Magnet retention on a rotor may take into consideration several factors. Magnets may be made of a relatively brittle material, such as various ceramics, and may be structurally weak and centrifugal forces during operation may be high, particularly with high speed rotors. Further, it is desirable to minimize the space between the rotor and the stator for purposes of maximizing the magnetic field, which weakens as the radial separation between the rotor and the stator increases, and limiting leakage of magnetic flux there between. Moreover, permanent magnet motors frequently operate in environments spanning a wide range of temperatures, and therefore, the rates of thermal expansion of the components of the rotor may differ substantially over the temperature range.
In some systems, magnets have been bonded to the surface of a rotor using an adhesive or other substance, and then held in place by an outer tape of high-strength material, such as glass or carbon fibre. In such systems, an encapsulant may be used to fill the spaces between the magnets. Such a method may result in the expenditure of substantial time to fabricate the rotor. Further, the thickness of the tape may increase the distance between the stator and rotor, resulting in increased magnetic flux leakage and reduced efficiencies of the motor. Moreover, expansion rates associated with the tape under tension and fluctuations in temperature may make it difficult to maintain an adhesive bond in compression at high rotational speeds. In the absence of such compression, the adhesive bond may peel, thereby allowing the magnets to move.
In other systems, magnets have been placed inside of the rotor, such that the rotor structure retains the magnets. For example, U.S. Patent Publication No. 2008/0157620 to Longo et al. (“Longo”) discloses a process for mounting magnets in a rotor, the rotor comprising a plurality of axial channels and a plurality of inner radial projections. Within each of these axial channels is disposed a permanent magnet, which is retained via deformation of each of the plurality of inner radial projections which may form a stop to axially retain the magnets. Further, PCT Publication WO 01/06624 to Matsushita Electrical Industrial Co. Limited (“Matsushita”) discloses a synchronous motor including a stator, a rotor, and permanent magnets. The permanent magnets of Matsushita are butted end to end to form V type configurations and embedded internally in magnet retaining holes of the rotor. However, the interior magnet constructions of the prior art may result in compromises in the magnetic circuit that may reduce performance and efficiency. Further, using these interior magnet designs may lead to complications in optimizing the distance between the stator and the rotor magnets, among other issues.
In yet other systems external systems for retaining magnets have been implemented. For example, U.S. Patent Publication No. 2007/0222317 to Morel (“Morel”) discloses a synchronous motor including a rotor with a plurality of magnets with corresponding clamping elements arranged as a web between the magnets and the body of the rotor. Following insertion of each of the magnets associated with the rotor, a tool or punch must be utilized on each of the clamping elements to deflect the web of the elements such that they contact the magnet. Such systems may result in substantial added time during assembly and further proper assembly may depend on the skill of an operator actuating the tool. Further, such systems may suffer from “spring back” of the material used for forming the web. This spring back may result in poorly restrained magnets and possible early failure of the rotor.
In yet other systems, externally situated magnets have been retained via notches formed in the magnets and restraining elements lockingly engaged in these notches. Such a system is disclosed in U.S. Pat. No. 6,732,986 to Heidrich (“Heidrich”). Such systems may also result in added assembly time as a result of the desire to ensure locking engagement of the restraining elements with the magnet notches. Further, such configurations may limit the number of magnets to be implemented on a particular rotor, thereby resulting in potential efficiency reductions.
Japanese publications JP2008-109726 and JP2008-141799 describe rotors having a laminated core with a through hole and a plurality of substantially arc-like magnets disposed on the external circumference of the laminated core. German publication DE102005048731 describes recessed retaining overhangs extending in an axial direction with retaining sections, which are pre-stressed to sit close to permanent magnets.
Accordingly, there exists a need for a rotor that is relatively easy to assemble and reliable, has acceptable performance of the magnetic circuit and is capable of holding the magnets under a wide range of rotational speeds and temperatures, while further avoiding excessive stresses on the magnets.